In parent application No. 562,547, which is hereby incorporated by reference, a bellows assembly is described which is made up of a bellows and a retainer which coact to form a pair of uniformly spaced seals running the full length of the bellows. The bellows itself is made up of a sheet of film with two extrusions suitably secured thereto at a uniformly spaced distance to form an integral unit. Accordion pleat members located at the two ends complete the seal. The two extrusions run the length of the film and, in cross-section, are of an arrowhead or mushroom configuration and deform to permit attachment of the film member to the retainer. The bellows or bladder coacts with a cut off plate to form a damper. The air flow was over the full width bellows for its entire length and there was no directional change in flowing over the bellows other than that incidental to the flow regulating valving action due to bellows inflation/deflation.
The above described damper provides a uniformity of devices such that it was no longer necessary to calibrate every terminal. This design has flow volume limitation due to the relatively limited width of the single flow path at the full open position. By locating the bellows transversely to the flow, two controlled flow paths or outlets are formed. This modification requires the air to impinge upon the bellows with the air being diverted into two flow paths. The result is a noisier unit. Placing porous foam on the surface impinged upon by the air flow has a sound reducing effect but the foam provides a leakage path which makes shutting the damper essentially impossible when using duct pressure to inflate the bellows since the same pressure is on both sides of the bellows. Also, the amount of leakage varies with the degree of compression of the foam subsequent to the foam contacting the cutoff plates.